1. Field of the Invention
The invention relates to a sensor device for determining the deviation of the position of a head support of a motor vehicle seat from a target position.
2. Description of the Background Art
In a motor vehicle seat, head supports are used to support the head of a vehicle occupant using the seat in the event of a crash and to thereby prevent the cervical spine from snapping backward. However, the full extent of the increased safety achieved by the head support is realized only if the head support is adjusted to the correct height with regard to the vertical head position so that, in the event of a crash, the head largely strikes the center of the head support provided for this purpose. An incorrectly adjusted head support, on the other hand, may cause injuries in the neck and head area in the event of a crash. To minimize the risk of head injury, for example a concussion when the head strikes the head support, the head support should also be situated as close as possible to the vehicle occupant's head in the horizontal direction and yet enable unobstructed head movement.
To correctly position a head support, the head support is usually adjustable in the vertical direction and, to a certain extent, also in a horizontal direction with regard to the seat backrest. In a high-end motor vehicle seat, the adjustment of the head support is frequently motor-driven.
To avoid incorrect positioning of the head support, a modern head support adjusting device is sometimes assigned a sensor device which detects the head position of a vehicle occupant with regard to the associated head support. For example, a head support adjusting device known from EP 1 857 318 A2, which corresponds to U.S. Pat. No. 7,967,377, is assigned a sensor device in the form of a capacitive proximity sensor which is integrated into the head support. The proximity sensor includes three electrodes which are disposed at a distance from each other in the vertical direction on the front of the head support. The two outer electrodes are used as transmitting electrodes for transmitting an alternating electric field in a spatial area situated in front of the head support. The middle electrode is used as a common receiving electrode for measuring the capacitance formed between each of the transmitting electrodes and the receiving electrode. The known adjusting device utilizes the physical effect that the capacitance of the electrode arrangement varies in a characteristic manner when the head is present in the alternating electric field. The deviation of the head support position from a predetermined vertical target position with regard to the head of the vehicle occupant is determined by comparing the capacitance values assigned to each of the two transmitting electrodes. The head support position at which these capacitance values are identical is detected as the target position for the head support.
Similar sensor devices which have a capacitive proximity sensor which includes three electrodes for detecting the head position relative to a head support are furthermore also known from FR 2 884 775 A1 and DE 199 16 804 C1, which corresponds to U.S. Pat. No. 6,402,195.